This invention relates to apparatus for, and a method of, detecting error conditions in electro-optic displays and their associated drive circuitry.
In certain applications, such as the dispensing of fuel, stringent requirements as to the accuracy and integrity of the display components are laid down. In particular the relevant authorities stipulate that a system of error detection be incorporated to warn the operator of a display error or to halt the dispensing of fuel when such a condition is detected.
Within an electro-optic display and associated drive circuitry, there are a number of errors or fault conditions that can arise including: (i) open or short circuits in connections between the driver devices and electrodes of the display; (ii) open or short circuits within the display electrodes or their interconnections; or (iii) failure of the driver devices resulting in the display's electrodes being driven by incorrect drive levels. Such conditions could result in a display which appears to be operating correctly when in fact it is indicating an incorrect value.
Various error detection schemes have formerly been proposed including methods for optically scanning the display or monitoring the electrode voltage via additional connections to the display electrodes. Such schemes tend to be complex and relatively expensive due to the additional circuitry required. Also if an additional connection is required then a special display has to be used.
Other methods provide checking of the input data or drive levels to the display electrodes; however, such an arrangement cannot take account for failures within the display device itself.
This invention provides error detection apparatus for an electro-optic display of the type in which drive circuitry applies a drive voltage across a capacitive display element, the apparatus comprising means for electrically isolating an element from the drive circuitry, means for monitoring a potential difference across the element when it has been isolated and means for generating an error signal if the potential difference is below, or in a pre-determined period decays below, a reference voltage.
The invention may advantageously be employed with liquid crystal or electro-luminescent displays.
By employing the present invention it is possible to detect errors arising out of open or short circuit failures in the drive circuitry as well as open or short circuit failures within the electro-optic display, or the connections thereto.
Advantageously the drive voltage is applied across drive electrodes of the element, and the potential difference across the drive electrodes is monitored. This enables the present invention to be used with any conventional electro-optic display of the type mentioned.
Preferably the isolating means is provided by the drive circuitry which includes a switchable high impedance output state. Once set to this state the potential difference across the element can be monitored.
The present invention lies in the realization that error detection can be implemented by exploiting the capacitance of the display element and monitoring the potential difference on the display's drive electrodes. However, small parasitic capacitances may, in some applications, be associated with the drive or test circuitry, and this may maintain sufficient voltage to mask any potential difference associated with the display element. This can be a problem with open circuit faults in the connections to the display or open circuit faults within the display itself. To overcome this a resistor is preferably connected across the element. The value of the resistor is selected to ensure that, in the absence of a fault condition, the potential difference across the combined parasitic and display element capacitance does not decay below the reference voltage in the pre-determined period. However the parasitic capacitance alone (i.e. in an open circuit fault condition) is not sufficient to prevent the voltage decaying below the reference voltage in the pre-determined period, and therefore employing the resistor avoids the parasitic capacitance masking a fault condition.
Where it is required to be able to detect open circuit fault conditions even when the display is in an "off" state (i.e. when there is no drive voltage across the display element), it is desirable to connect a first end of a resistor to an electrode of the display element and the second end to a bias voltage. The bias voltage may be chosen to be an inverse of the drive voltage or a fraction of the drive voltage. With such an arrangement the potential difference across the element will tend towards the bias voltage when the element is isolated.
Advantageously the potential difference is monitored using a voltage comparator having a first input connected to an electrode of the element and a second input connected to the reference voltage. The reference voltage is preferably a fraction of the drive voltage.
In certain types of electro-optic displays, for example liquid crystal displays, it is preferable to drive the display dynamically by cyclically switching the drive voltage polarity to maintain the display element in a given state. Such a drive arrangement prevents polarisation of the liquid crystal. When the drive voltage polarity is cyclically switched the potential difference should be monitored between switching transitions. Furthermore it is desirable to successively monitor the potential difference on opposite drive voltage polarities, so that the process of monitoring does not polarise the liquid crystal.
According to a second aspect of the invention there is provided a method for detecting errors in an electro-optic display comprising applying, by means of drive circuitry, a drive voltage across a capacitive display element; isolating the element from the drive circuitry; monitoring a potential difference across the element when it has been isolated; comparing the monitored potential difference with a reference voltage; and generating an error signal if the potential difference is below, or in a pre-determined period decays below, a reference voltage.